The idea that caloric restriction slows the aging process for some organisms is nearly as well accepted as the inevitability of aging. In yeast and worms, life extension due to calorie restriction involves the protein deacetylase Sir2. In 2001, Leonard Guarente's lab at the Massachusetts Institute of Technology showed that the histone deacetylase Sir2 needs the forkhead family transcription factor Daf-16 to extend worm life. It was a big clue, says Guarente; someone just needed to put the pieces together in mammals.
In the Hot Papers featured here, several labs made a convincing case that SIRT1 regulates forkhead transcription factors (FOXOs) in mammals. Guarente's group showed that SIRT1 directly deacetylates FOXO3, resulting in the downregulation of apoptotic genes.1 Michael Greenberg's group at Children's Hospital in Boston showed that SIRT1 binds FOXO3 and deacetylates it, resulting in the activation of DNA-repair genes.2
The seemingly opposite...
Anne Brunet, first author on the Greenberg paper and now at Stanford University, says that in 2004, labs interested in aging, cancer, and neuroprotection were all focusing on SIRT1 and FOXO proteins. "It was a race," says Brunet, "a friendly race, but nevertheless, a race to connect the two."
The idea was that SIRT1 might provide a mechanism for controlling the function of FOXO proteins through deacetylation. FOXO transcription factors were well known to be involved in longevity through the insulin-signaling pathway, as well as in controlling cell-cycle arrest and DNA repair. Nevertheless, "SIRT1 was relatively a new player in the field," says Brunet, "and it was intriguing to try and uncover its mammalian substrates."
SIRT1 was already known to deacetylate histones, MyoD, and p53, but FOXO had an extra wrinkle of interest due to its effects on longevity. It therefore made a splash when the papers showed that acetylation could regulate FOXO proteins in mammalian cell lines, says Su-Ju Lin of the University of California, Davis. "But a lot of details are still unclear on how SIRT1 is affected by calorie restriction," says Lin.
Shortly after the two labs published, Boudewijn Burgering's lab at the University Medical Center of Utrecht published evidence that FOXO proteins are regulated by reversible acetylation, in part involving SIRT1 deacetylation.3 Michael McBurney of the Ottawa Health Research Institute says it wasn't a huge leap to test the SIRT1-FOXO connection, so it's not a big surprise that three labs published simultaneously. "But what was more interesting was that they got different answers," he says.
Indeed, while in each case SIRT1 deacetylated FOXO3, Guarente's lab found that this decreased the activity of FOXO3,1 whereas Greenberg's lab found that this increased its activity.2 To be fair, the labs were looking at apoptotic and DNA-repair genes, respectively. Plus, "it's not out of the question that SIRT1 could be both inhibiting apoptosis and activating DNA repair," says Brunet, "but exactly how, is an unanswered question."
While the exact mechanism for how SIRT1 connects FOXO to aging is still being explored, it has emerged that SIRT1 does a surprising number of things in mammals. For example, SIRT1 appears to promote fat mobilization into the blood via the peroxisome proliferator-activated receptor (PPAR) g, affect neuronal integrity, and contribute to insulin sensitivity. "As such a global regulator, SIRT1 is the best candidate to mediate the salutary effects of calorie restriction on health and longevity," says Guarente.
Promiscuous and Cancerous
However, McBurney suggests that what is a substrate for SIRT1, and what isn't, is not as clear as two years ago. For example, McBurney's lab recently reexamined the SIRT1/p53 connection in SIRT1-/- mice.4 They found that while SIRT1 and p53 interact, this has little effect on p53-mediated functions. "This brings up the question of whether we really know what the relevant [in vivo] substrates of SIRT1 are," he says, "since it appears to be fairly promiscuous [in vitro]."
This pleiotropic nature gets even more complicated, as recent results from Stephen Baylin's lab at John Hopkins reveal that SIRT1 affects the phenotypes of cancer cells by localizing to the promoters of epigenetically-silenced genes.5 "This raises the fascinating possibility that SIRT1 is an oncogene," says Baylin, "since it appears to participate in epigenetic gene silencing."
It all points to a balance, in that modulating SIRT1 could be good for longevity, but dangerous in terms of cancer risk. Experiments in mice, underway in a number of labs, should get at a big question of whether longevity and cancer can be uncoupled, says Brunet. But answers will take time, since mice live longer than yeast and worms. "It's another race," says Brunet, "just on a longer time scale."
References1. M.C. Motta et al., "Mammalian SIRT1 represses forkhead transcription factors," Cell, 116:551-63, Feb. 20, 2004. (Cited in 135 papers, Hist Cite Analysis)2. A. Brunet et al., "Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase," Science, 303:2011-5, March 26, 2004. (Cited in 167 papers, Hist Cite Analysis)3. A. van der Horst et al., "FOXO4 is acetylated upon peroxide stress and deacetylated by the longevity protein hSir2(SIRT1)," J Biol Chem, 279:28873-9, July 9, 2004. (Cited in 52 papers)4. C. Kamel et al., "SirT1 fails to affect p53-mediated biological functions," Aging Cell 5:81-8, February 2006.5. K. Pruitt et al., "Inhibition of SIRT1 reactivates silenced cancer genes without loss of promoter DNA hypermethylation," PLoS Genetics 2:e40, March 2006.
Data derived from the Science Watch/Hot Papers database and the Web of Science (Thomson Scientific, Philadelphia) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age. M.C. Motta et al., "Mammalian SIRT1 represses forkhead transcription factors," Cell, 116:551-63, Feb. 20, 2004. (Cited in 135 papers, Hist Cite Analysis) A. Brunet et al., "Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase," Science, 303:2011-5, March 26, 2004. (Cited in 167 papers, Hist Cite Analysis)